π-Conjugated polymers as a class have in recent decades been the subject of numerous publications. They are also referred to as conductive polymers or as synthetic metals.
Conductive polymers are gaining increasing economic importance, since polymers have advantages over metals in terms of processibility, weight and the targeted setting of properties by means of chemical modification. Examples of known π-conjugated polymers are polypyrroles, polythiophenes, polyanilines, polyacetylenes, polyphenylenes and poly(p-phenylene-vinylenes). Layers of conductive polymers are widely used in industry. A review may be found in L. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik & J. R. Reynolds, Adv. Mater. 12 (2000) 481-494.
Conductive polymers are prepared by chemical oxidation or electrochemically from precursors for the preparation of conductive polymers, e.g. substituted or unsubstituted thiophenes, pyrroles and anilines and their possibly oligomeric derivatives. Polymerization by chemical oxidation is particularly widespread because it can be applied in a technically simple fashion to a variety of substrates. For this purpose, the precursors for the preparation of conductive polymers are polymerized by means of an oxidant. The polymerization is so fast that the precursors for the preparation of conductive polymers and the oxidant generally have to be applied to the substrate one after the other. However, a problem with this sequential application is that stoichiometric ratios between the precursors for the preparation of conductive polymers and the oxidant can be set only with great difficulty. As a result, the reaction to form the polymer is incomplete, the precursors are utilised only incompletely and the quality of the conductive layer and its conductivity are decreased.
Furthermore, the sequential application multiplies the number of process steps necessary, so that sequential processes are associated with significantly higher process costs. It is therefore desirable to use precursors for the preparation of conductive polymers and the oxidant together and in precisely defined mixtures.
Mixtures of oxidants and precursors for the preparation of conductive polymers have sufficiently low reaction rates to be able to be employed in industrially useable processes only at low temperatures. Thus, for example, in U.S. Pat. No. 5,455,736, a dilute mixture of pyrrol and oxidant is cooled to low temperatures in order to slow the polymerization sufficiently. However, the use of low temperatures is firstly very complicated in engineering terms and, secondly, the solubility of the oxidant is limited at low temperatures and the viscosity of the solution increases greatly with decreasing temperature. A further disadvantage is that the low temperature results in moisture from the surrounding air getting into the cooled solutions and the quality of the conductive polymers prepared from the solutions being altered in a disadvantageous manner.
EP-A 339 340 describes the polymerization of 3,4-disubstituted thiophenes by chemical oxidation. If the oxidant is selected appropriately, these thiophenes can also be processed in solution in the presence of the oxidant to produce conductive layers. However, here too, the reaction commences after only a few minutes.
EP-A 615 256 states that the polymerization in mixtures of oxidants and precursors for the preparation of conductive polymers can be slowed by addition of a non-volatile base such as imidazole. In this way, the polymerization can be suppressed for a few hours. However, the additive remains in the conductive layer and can there have an adverse effect on the function of the layer.
In U.S. Pat. No. 6,001,281, the polymerization is slowed by use of two solvents having different boiling points. The more volatile solvent is chosen so that it forms a weak complex with the Fe(III) used as oxidant and thus slows the reaction. On the other hand, the solvent having the higher boiling point does not complex the Fe(III). To carry out the polymerization, the more volatile solvent is evaporated, after which the reaction proceeds in an accelerated fashion. This method has the great disadvantage that the reactive solution has to be strongly diluted with a further solvent. Furthermore, the solvents used, e.g. tetrahydrofuran, are industrially undesirable.
There is therefore a continuing need for oxidants which can be employed together with precursors for the preparation of conductive polymers at temperatures which can easily be managed industrially, with the polymerization being suppressed for a time which is sufficiently long for industrial applications without complicated further process steps having to be carried out to prevent it.
It is therefore an object of the invention to find and prepare suitable oxidants for the polymerization of precursors for the preparation of conductive polymers by chemical oxidation, which oxidants suppress the polymerization for a sufficiently long time and by means of which conductive layers, for example for solid electrolyte capacitors or other applications, can be produced.